1. Field of the Invention
The invention relates to a DC to AC inverter and, in particular, to a DC to AC inverter that can reduce switching loss and increase system efficiency and a negative input terminal of a DC power input port can be directly connected to the neutral line of a single-phase utility system.
2. Description of Related Art
The wide use of fossil fuels has resulted in the problem of greenhouse emissions worldwide, which has also seriously damaged the earth's environment. Moreover, fossil fuels will be exhausted in the future, and the cost of fossil fuels has significantly increased recently. Governments worldwide expect to develop renewable energy to alleviate the above problems. Therefore, it has become a trend to look for green energy in place of fossil energy. This results in the interest in developing solar power, wind power, and fuel cells. The grid-connected solar power system and wind power system are the important renewable power systems currently.
Taking the grid-connected solar power system as an example, the solar power system basically has a DC to DC converter, a DC to AC inverter and a controller. First, the DC to DC converter converts the power generated by the solar cell array to DC power. Then, the DC to AC inverter converts the DC power into AC power, and the AC power is then injected into the utility system. Therefore, a suitable circuit topology for the DC to AC inverter can increase the efficiency of entire power system, minimize the size, and reduce the cost.
With the advance in technology of semiconductor devices, many power electronic switches with high switching frequencies have been developed and used in power conversion equipment. However, these power electronic switches have switching loss and therefore reduce the efficiency. It is thus very important to design a circuit topology that can reduce switching loss and increase equipment efficiency for green energy power systems.
The design of the DC to AC inverter is very important for renewable energy generation systems. In a grid-connected renewable energy generation system, the DC to AC inverter is used to convert DC power into AC power and inject the AC power to the utility system. The output current of DC to AC inverter is a sinusoidal current that has the same phase as the utility system. The DC to AC inverters generally can be categorized as a half-bridge configuration and a full-bridge configuration. With reference to FIG. 1, a conventional half-bridge inverter mainly comprises a first power electronic switch TA+, a second power electronic switch TA−, and two equivalent DC capacitors. When the first power electronic switch TA+ is turned on and the second power electronic switch TA− is turned off, the output voltage of the inverter is
      V    O    =      +                            V          d                2            .      On the other hand, when the second power electronic switch TA− is turned on and the first power electronic switch TA+ is turned off, the output voltage of the inverter is
      V    O    =      -                            V          d                2            .      Therefore, turning the first power electronic switch TA+ and the second power electronic switch TA− on and off alternately can control the inverter to output an AC voltage and thus a desired AC current. To achieve this purpose, Vd/2 has to be greater than the voltage amplitude of the utility system.
With reference to FIG. 2, a conventional full-bridge inverter comprises a first power electronic switch TA+, a second power electronic switch TA−, a third power electronic switch TB+ and a fourth power electronic switch TB−. When the first power electronic switch TA+ and the fourth power electronic switch TB− are turned on while the second power electronic switch TA− and the third power electronic switch TB+ are turned off, the output voltage of the inverter is VO=+Vd. On the other hand, when the second power electronic switch TA− and the third power electronic switch TB+ are turned on while the first power electronic switch TA+ and the fourth power electronic switch TB− are turned off, the output voltage of the inverter is VO=−Vd. When the first power electronic switch TA+ and the third power electronic switch TB+ are turned on while and the second power electronic switch TA− and the fourth power electronic switch TB− are turned off, or the second power electronic switch TA− and the fourth power electronic switch TB− are turned on while the first power electronic switch TA+ and the third power electronic switch TB+ are turned off, the output voltage of the inverter is VO=0. Generally speaking, Vd has to be greater than the voltage amplitude of the utility system. Therefore, by controlling the first power electronic switch TA+, the second power electronic switch TA−, the third power electronic switch TB+, and the fourth power electronic switch TB− to turn on and off alternately, the inverter is controlled to output an AC voltage. Thus, a desired AC current can be output from the inverter.
For either full-bridge or half-bridge inverters, at lest two power electronic switches are switching in high-frequency at the same time. Therefore, there is a lot of switching loss. In a single-phase utility system, one of the power lines is the neutral line. In some application of renewable energy generation systems, such as that thin-film solar cell using amorphous-Si, the output voltage of solar cell array is directly connected to a DC power input port of the inverter. In practice, a negative input terminal of the DC power input port of the inverter has to be connected to the neutral line of the utility system. In the half-bridge or full-bridge inverter as shown in FIG. 1 or FIG. 2, the negative terminal on the DC side cannot be directly connected to the neutral line of the utility system. Therefore, when the half-bridge or full-bridge inverter is used in some of the renewable energy generation systems, an additional transformer is required for galvanic isolation. This results in increasing the cost and size, and decreasing the efficiency. Consequently, it is very important to develop a grid-connected inverter that has high efficiency and can directly connect the negative input terminal of its DC power input port to the neutral line of the utility system.